1. Field of the Invention
The present invention relates to a method of forming group III-N HEMTs and, more particularly, to a method of forming metal contacts in the barrier layer of a group III-N HEMT.
2. Description of the Related Art
Group III-N high electron mobility transistors (HEMTs) have shown potential superiority for power electronics due to their wider bandgap and high electron saturation velocity. These material properties translate into high breakdown voltage, low on-resistance, and fast switching. Group III-N HEMTs can also operate at higher temperatures than silicon-based transistors. These properties make group III-N HEMTs well suited for high-efficiency power regulation applications, such as lighting and vehicular control.
A conventional group III-N HEMT includes a substrate, and a layered structure that is formed on the top surface of the substrate. The layered structure, in turn, includes a buffer layer that lies on the substrate, a channel layer that lies on the buffer layer, and a barrier layer that lies on the channel layer. Further, the layered structure can optionally include a cap layer that lies on the barrier layer.
The buffer layer provides a transition layer between the substrate and the channel layer in order to address the difference in lattice constant and to provide a dislocation-minimized growing surface. The channel layer and the barrier layer have different polarization properties and band gaps that induce the formation of a two-dimensional electron gas (2DEG) that lies at the top of the channel layer. The 2DEG, which has a high concentration of electrons, is similar to the channel in a conventional field effect transistor (FET). The cap layer enhances the reliability of the group III-N HEMT.
A conventional group III-N HEMT also includes a metal gate that is formed on the top surface of the layered structure. The metal gate makes a Schottky contact to the barrier layer (or the cap layer if present). Alternately, the metal gate can be isolated from the barrier layer (or the cap layer if present) by an insulating layer.
In addition, a conventional group III-N HEMT includes a source metal contact and a drain metal contact that lies spaced apart from the source metal contact. The source and drain metal contacts, which lie in metal contact openings that extend into the layered structure, make ohmic contacts with the barrier layer.
Native group III-N substrates are not easily available. As a result, group III-N HEMTs commonly use a single-crystal silicon substrate. (Silicon carbide is another common substrate material for group III-N HEMTs.) The layered structure is conventionally grown on the substrate using epitaxial deposition techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).
Each of the layers in the layered structure is typically implemented with one or more sequential group-III nitride layers, with the group-III including one or more of In, Ga, and Al. For example, the buffer layer can be implemented with sequential layers of AlN (a thermally-stable material), AlGaN, and GaN. In addition, the channel layer is commonly formed from GaN, while the barrier layer is commonly formed from AlGaN. Further, the cap layer can be formed from GaN.
The source and drain metal contacts are conventionally formed by forming a passivation layer, such as a silicon nitride layer, on the top surface of the layered structure (on the top surface of the cap layer if present, or the top surface of the barrier layer when the cap layer is not present). Following this, a patterned photoresist layer is formed on passivation layer.
After the patterned photoresist layer has been formed, the exposed regions of the passivation layer, the underlying portions of the cap layer (if present), and the underlying portions of the barrier layer are dry etched for a predetermined period of time using a gas combination that includes CHF3, CF4, Ar, and O2.
The dry etch forms source and drain metal contact openings that extend through the passivation layer, through the cap layer (if present), and into the barrier layer. It is very difficult to control the depths of the metal contact openings because the etch is very short, typically a few seconds. As a result, the bottom surface of the metal contact openings frequently extends through the barrier layer and into the channel layer.
After this, a metal layer is deposited to lie over the passivation layer and fill up the metal contact openings. The metal layer is then planarized to expose the top surface of the passivation layer and form source and drain metal contacts in the source and drain metal contact openings, respectively.